Method of recovering oil from underground reservoir



XR @9209 Q24 METHOD OF RECOVERING OIL FROM UNDERGROUND RESERVOIR Filed Jan. 21, 1963 2 Sheets-Sheet 1 w 4 I w N w 2 H S R ISM A N W E C O ..T O B {WES R R N R M D A U Y C B S N H O a -mo R A D 0 .RM Y O I P 1 H R 6 A D RC 9/ 7 V A0 0 H l 4 B R a N D C Y D H o 2N O G m O 6 2 8 4 O 6 2 8 v 4 O M 4 3 3 2 2 2 l I 22:98 ZOE/Some: 2 609 X 511E335 55232025 9 8 7 6 3 3 3 3 3 09; 51 z PER CENT N2 IN N2 NATURAL GAS MIXTURE INVEN TORS HOWARD A KOCH, JR. LOYD R; KERN d. 5, 1965 n KOCH; JR" ETAL, 3,209,824

METHOD OF RECOVERING OIL FROM UNDERGROUND RESERVOIR Filed Jan. 21, 1963 2 Sheets-Sheet 2 a LU 0: I00 LL! 8 v L AST 22 FEET 90 4 Lu I FRST I23 FEET I z I 1 O as as 37 3a 39 4o DISPLACEMENT PRESSURE, PSI X I00 Fig. 3

u so

m E z 50 P Lu 2 (.7 if a; 40 2 Lu 0 CL 0 3o O E E 20 (D 2,

n: *1 IO z LITERS FREE GAS PRODUCED Fi 4 INVENTORS HOWARD A. KOCH, JR. BY LOYD R. KERN @nited g 3,209,824 METHQD OF RECOVERZNG OIL FROM UNDERGROUND RESERVOIR Howard A. Koch, Jr., Dallas, and Loyd R. Kern, Irving, Tom, assignors to The Atlantic Refining Company, Philadelphia, Pin, a corporation of Pennsylvania Filed Jan. 21, 1953, Ser. No. 252,641

Claims. (Cl. 166-9) In the recovery of oil from subsurface earth formations by the application of external energy, as opposed to natural reservoir energy, it has heretofore been proposed to inject natural gas and other gases which are sparingly soluble in reservoir oil under conventional reservoir pressure or nitrogen, air or other gases which are inert so far as their action or reaction with reservoir oil is concerned and are substantially insoluble therein. At the conventional pressures employed in such operations, generally below about 1500 psi, any fluid in its gaseous state merely acts as a pusher; and, due to vast differences between the mobility of the gas as compared with the mobilityof the more viscous oil, substantial amounts of the oil are bypassed and cannot be recovered. In addition, nitrogen, air, or other gases containing inert components, are much less efiicient than natural gas since inert gases are substantially less soluble in reservoir oil than is natural gas.

It is, therefore, an object of the present invention to provide an improved method of recovering oil from a non-depleted oil reservoir by the injection into the reservoir of gases comprising nitrogen with minor amounts of other gases at a pressure within a critical range.

Another object of the present invention is to provide an improved method of recovering oil from a nondepleted oil reservoir by the injection of gases comprising nitrogen with minor amounts of other gases at a pressure below that pressure which would .fracture the subsurface formation and above a pressure which will maintain the injected gas and reservoir oil miscible at all points of contact.

Another and further object of the present invention is to provide an improved method for producing oil from a nondepleted oil reservoir by the injection of a gas containing at least about 80 percent by volume of nitrogen at a pressure below that pressure which will fracture the subsurface formation but above a pressure which will maintain the injected gas and reservoir oil miscible at all points of contact.

A further object of the present invention is to provide an improved method for producing oil from a nondepleted oil reservoir by the injection of nitrogen, air, flue gas or other gases containing at least about 80 percent by volume of nitrogen at a pressure below that pressure which will fracture the formation but above a pressure which will maintain the injected gas and reservoir oil miscible at all points of contact.

A still further object of the present invention is to provide an improved method for recovering oil from a nondepleted oil reservoir by the injection of a gas containing at least about 80 percent by volume of nitrogen, in which less than the first 0.10 pore volume of such gas initially contains at least about 90 percent by volume of C; to C res .batent Q 3,209,824 Patented Oct. 5, 1965 ice hydrocarbons gradually decreasing to 0 percent, at a pressure above that pressure which will maintain the injected gas and the oil miscible at all points of contact.

Other and further objects of the present invention will be apparent from the following detailed description when read in conjunction-with the drawings,.wherein;-

FIGURE 1 is a plot of crioondenbar pressures for methane-0 hydrocarbon systems versus cricondenbar pressures for nitrogen-(1 hydrocarbon systems,-

FIGURE 2 is a plot of percent by volume of nitrogen in a nitrogen-natural gas mixture versus miscible displacement pressure,

FIGURE 3 shows plots of percent oil displaced from the first 123 feet and the last 22 feet of a 145 foot core by flue gas at various pressures, and

FIGURE 4 is a plot of the nitrogen content of the free gas produced after breakthrough of displacement gas versus the volume of free gas produced during the disj placement of oil from a 145 foot core by flue gas at 3850 p.s.r.

Although, classically, the term cricondcnbar has been used to denote the maximum pressure at which a given binary system of a given composition can'exist in two phases (Chemical Engineering Thermo-Dynamics, Dodge- McGraw-Hill, first edition, page 545), for purposes of the present invention, the term, crieondenbar, shall mean the maximum pressure at reservoir temperature at which two phases can exist for any mixture consisting of the reservoir oil and the injection gases proposed to be injectcd into the reservoir. An example of the cricondenbar pressure as used in this sense is disclosed in Petroleum Transactions, AIME, volume 195, page 181, 1952.

The terms nondepleted reservoir or producing reservoir as used herein are meant to include an oil reservoir which, at the beginning of treatment by the method of this invention, contains reservoir oil which is undersaturated with gas or an oil reservoir which, at the beginning of treatment by the method of this invention, contains reservoir oil saturated with gas and a small volume of free gas but which, upon application of the preselected pressure in accordance with the present invention, will become undersaturated by solution in the oil of the free gas phase.

It has been taught in United States Patents Nos. 2,724,- 437 and 2,7 24,438 that substantially improved oil recovery can be obtained by injecting into a nondcpleted reservoir natural gas or other normally gaseous hydrocarbons at pressures which will maintain miscibility between the injected gas and the reservoir oil. In some instances, such miscibility canbe attained by injecting the natural gas at pressures above the cricondenbar pressure provided the formation can withstand such high pressure without formation fracturing occurring. However, as specifically taught in accordance with United States Patent No. 2,724,-

same ultimate production can be obtained at these lower pressures as can be obtained at pressures above the cricondenbar pressure.

In view of the prior art disclosures that nitrogen and gases containing substantial amounts of nitrogen can be employed in low pressure gas injection processes coupled with the teachings of United States Patent No. 2,724,438, one would assume that nitrogen could be substituted directly for normally gaseous hydrocarbons in the miscible displacement process of the subject patent. However, based on all available data concerning nitrogen-hydrocarbon systems, it becomes rather obvious that such substitution is not practical. Based on available data concerning cricondenbar pressures for nitrogen and various C, hydrocarbons and available data concerning the crithe spread between the two cricondenhar pressures for the same hydrocarbon becomes even greater. Based on the plot of FIGURE 1-, one may predict the cricondenbar pressure for 21 given reservoir oil based on the known or measured cricondenbar pressure for the same oil and methane. For example, for an undersaturated. reservoir fluid having the following composition:

Weight percent- C .11.; c 6.2 (3 6.6 c 5.8 c 3.5 c 5.1 c c 615 able to obtain miscibility by injecting nitrogen into a reservoir at or above the cricondenbar pressure.

However, as taught in United States Patent No. 2,724",-

438-, miscibilityat all points of contact between aninjected-natural gas and a reservoir oil can be attained at pressures well below the cricondcnbar pressure and ultiously set forth. This miscible pressure is approximately 7 60 percent lower than the cricondenbar pressure for the same system. Using these teachings of the subject patent, one could, therefore, predict that miscibility could be. attained between the reservoir fluid referred to and nitrogen at a pressure somewhat above 14,000 p.s.i. This predicted miscible pressure forthe nitrogen-oil system is, however, little help since, again, the pressure is above all practical limits.

Contrary to these teachings'of the prior art and the conclusions which one skilled in the art would reach based on these teachings, it has been found in accordance with the present invention that nitrogen or gases containing substantial amounts of nitrogen can be employed to miscibly displace reservoir oil from nondepleted. reservoirs at pressures only slightly greater than those necessary to attain miscible displacement with normally gaseous hydrocarb0ns., Employing the same reservoir oil previously exemplified, it has been found in accordance with the present invention that miscibility can beattained and a miscible displacement process carried out by injecting pure nitrogen at a pressure of 3870 psi. For this particular system, the difference between the miscible pressure for natural gas and that for pure nitrogen is only 370 p.s.i., and this miscible pressure for nitrogen is 10,130 p.s.i. lower than the miscible pressure which one would expect.

Work in connection with the process of United States Patent No. 2,724,438 and related miscible displacement processes had also taught that miscibility between the reservoir oil and the injected gas could be created within a few feet of travel of the injected gas and the miscibility pressure could in all cases be determined in a laboratory core 25 feet in length. In the patented process it was further taught that miscibility was created by a transfer of to C hydrocarbons from the reservoir oil to the leading edge of injected gas thereby building up a short transition zone in which the fluids at any given point were miscible with one another; In addition, it was well settled that thc'gas in contact'with-the oilmust be a hydrocarbon gas and that at least 0.1 pore volume of such gas .must be injected before any otherfluid in order to carry out the process and displace oil at practical reservoir pressures. v

It has also been found, in accordance with the present invention, that miscibility between oil and nitrogen or gases containingsubstantial amounts of nitrogen is not created until the gas-has traveled at least about 100 feet and the miscibility pressure in this case cannot bedetermined in a core less than about 100 feet in length. The comparatively long path length required to create miscibility between nitrogen and oil is attributable to the additional discovery that nitrogen must be transferred from the injected gas to the oil. and. C in addition to C -C hydrocarbons, must betransferred from the oil to the injectcd gas to build up a transition zone with miscibility throughout.

Therefore, in accordance with the present invention, substantially improved results can be. obtained over conventional low pressure inert gas injection processes by injecting gases containing at least about percent by volume. of nitrogen into the reservoir at a. pressure in excess of that necessary to-maintain miscibility, as determined by a conventional laboratory procedure hereinafter described, but below that pressure which will cause. fracturing. pf therescrvoir't'ormation. i

FIGURE 2 illustrates the unexpectedly low pressures at which miscibility can be. attained when nitrogen or mixtures of nitrogen with C areused to displace the previously mentioned. reservoir oil as compared with pure C In this series of runs C nitrogen and mixtures of C and nitrogen were used to displace the oil from an artificial sandv column 123 feet in length at a temperature of 140 F. The ultimate stock tank recovery at abandonment gasoil ratio (30,000 sci/STE) was calculated as percent of the oil initially in place-in the core. The percent ultimate recovery for mixtures containing at least 80 percent nitrogen was found to range between 83.6 and 87.3 percent of 1 the oil initially in place. The difference between this re covery and the percent recovery obtained when math we or natural gas miscibly displaces oil will be apparent from the following discussion of FIGURE 3.

FIGURE. 3'plots the results of a series of tests conducted in the same manner as the tests of FIGURE 2- except that flue gas (-87.5 percent nitrogen and 12.5 percent CO was employed as the displacing gas in all cases and a detachable 22 foot section was added to. the 123 foot core to give anoverall length of 145 feet. In addition, in each test miscibility or lack of miscibility was determined at the end of the 123 foot core and the 22 foot extension and ultimate recoveries from the 123 foot core and the 22 foot. extension. were measured. These results are recorded in FIGURE 3'. It is to be observed that at 3600 psi. miscibility had not been attained by the time the gas had reached the end of either the 123 foot core or the 22 foot extension. At 3750 p.s.i. none of the oil had been miscibly displaced in the 123 foot core and only part of the oil was miscibly displaced in the terminal end of the 22 foot extension. At 3850 p.s.i. and 4000 psi part of the oil was miscibly displaced from the terminal portion of the 123 foot core and all of the oil was miscibly displaced from the 22 foot extension. Thus, it is obvious that gases containing substantial amounts of nitrogen must travel at least about feet in order to create miscibility and it a sufficiently long travel path is available miscibility can be attained at pressures only slightly above the miscible pressure of methane and the reservoir oil (3500 p.s.i.-). This length effect also explains the lower than anticipated recoveries for nitrogen as compared with methane since about percent of the oil was bypassed due to immiscible dis-' stantially 95 percent of the oil in place in the area'of the reservoir contacted.

In each of the runs of FIGURE 3, the free gas (gas in excess of solution gas) was analyzed after gas breakthrough at the end of the 145 foot core. FIGURE 4 a is a plot of the nitrogen content of the free gas produced in the3850 p.s.i. run.

It is to be observed that the gas in immediate contact with the oil contained 6.0 percent nitrogen, the remainderbeing C, through C hydrocarbons transferred from the oil. Thereafter, the nitrogen content of the free gas increased rapidly and the C through C hydrocarbons'decreased. It was further determined that in all cases the nitrogen content of the first gas was less than 1.0 percent and specifically between 3 and 7 percent. The ratio of C to C through C hydrocarbons in the free gas was also found to be between about 3 to 1 and 2 to 1. Thus, in order for nitrogen to misciblyldisplace oil at practical reservoir pressures the oil must be substantially free of nitrogen and there must be a transfer of nitrogen from the gas to the oil and C, from the oil to the nitrogen.

Although it was previously indicated that nitrogen must travel at least 100 feet in order to build upa transition zone with miscibility throughout, it-was found as a result of the previously described gas analysis that the transition zone (nitrogen mixed with C through C hydrocarbons) itself was only feet" long. Accordingly, in a variation of: the present invention the length of the path necessary to create miscibility can be reduced and the pressure necessary for miscibility can be reduced slightly by preinjecting the mixed transition zone. Specifically, less than the first 0.10 pore volume, and preferably 0.05 pore volume, of the injected nitrogen gas would have added thereto C through C hydrocarbons in a gradually decreasing volume from at least 90 percent to 0 percent. The added hydrocarbon gas should preferably'have a ratio of C, to C through C hydrocarbons of between about 3 to l and 2 to l. i

As stated previously, the upper limit of pressures to be employed in accordance with the present invention is that pressure which would be sufficient to fracture the oil-producing formation in which the process is to practiced. Generally, the formation breakdown pressure for formations deeper than 4000 feet consists predominantly ofv the pressure necessary to lift the overburden.

This formation breakdown pressure can be calculated with reasonable accuracy and is equal to between 0.57 and "0.85 p.s.i. per foot of depth for formations deeper than 4000 feet. At depths less than 4000 feet, the formation breakdown pressure is made up predominantly of the pressure necessary to overcome the rock-bonding strength which also can be calculated by well-known means but, in general is 4000 p.s.i. or less.

The minimum pressure in accordance with the present invention is that pressure at which miscibility between the reservoir oil and the injection gas exists as determined by laboratory long core displacement runs. Such long core displacement runs are described and illustrated in detail in the United States Patent No. 2,724,438 and briefly.consist of displacing reservoir oil from a core filled with such oil by means of the injection gas at a series of different pressures and observing through a visual cell the lowest pressure at which all effiuent is a single phase. At or above this miscible pressure, the effluent of the core is a single phase at all times and as observed through the visual cell the effluent changes 6 gradually from a single phase liquid oil before gas breakthrough to a single phase gas when a substantial change in the gas-oil ratio occurs (about 10,000 s.c.f./STB to 15,000 s.c.f./STB). As a practical matter, the run is usually continued until abandonment gas-oil ratio (gen erally selected at 30,000 s.c.f./STB of oil) is reached. In short, two simultaneously flowing phases of gas and liquid will not appear at any time during a miscible displacement; that is, at no time will gas bubbles appear in a liquid. phase, and at notime will liquid'dro'plets' appear in a gaseous phase.

In conducting long core displacement runs in accordance with this invention, an artificial core is constructed by filling a 100 foot or longer length of 2-inch tubing with a 140 to 200 mesh sand. The length of the sand core is important since a core shorter than 100 feet has been found to be too short to permit adequate mixing and contact at the zone of contact between the oil and the gas. Oil which has been stabilized at atmospheric pressure, which is generally referred to as dead oil, is then pumped into the core at a pressure of about 500 p.s.i. to

. displace any air or gas which might be present therein.

The core is then heated by a fluid-filled jacket oran elec trical coil to bring the temperature of the core to the desiredreservoir temperature. Reservoir oil having the same composition and characteristics as that of the reservoir to be treated is then pumped into the core until all of the dead oil has been displaced and the core is filled with reservoir oil at reservoir temperature and existing reservoir pressure. The injection gas, in this case nitrogen or mixtures of nitrogen with minor amounts of other gases, is pumped into the core at a preselected pressure while simultaneously producing fluids from the efiiuent end of the core. A back pressure is maintained on the effiuent end of the core'which is sufficient to maintain a pressure differential of approximately 50 p.s.i. so that oil will flow from the core due to the slightly higher pressure of the injection gas. Immediately adjacent the effluent end of the core, the effluent passes through a visual cell in which the operator may.=observe the nature of the eflluent at all times. By repeating this procedure at several different gas injection pressures, a pressure will be found at which the oil and the injection gas are miscible and at which a single phase will' be observed at all times throughout the course of the run. Below this pressure, two phases will be observed to be flowing simultaneously at some point during the run. Variations and modifications of this procedure which are well-known to those skilled in the art .may be practiced wtihout departing from the present invention. For example, a tube of smaller diameter may be employed so long as it is sutficicnt in length to permit adequate mixing and contact (at least 100 feet) or naphtha rather than dead oil may be employed to purge the core of air or gas.

While any pressure below formation breakdown pressure but above the miscible pressure, as determined above, can be employed in the practice of the present invention, 'it is obvious that the miscible pressure or a pressure slightly above the miscible pressure will be most convenient and economical.

As has been pointed out, the present invention is applicable to a producing or nondepleted reservoir, as defined,

since it is only in this type of reservoir that miscibility can be attained between the reservoir-oil and the injection gas and that the unexpectedly high recoveries of reservoir oil are possible. In depleted reservoirs which are not undersaturated or cannot be made undersaturated at the operating pressure, the injection gas will bypass a substantial amount of the oil and no advantage will be obtained over that of conventional low pressure displacement processes carried out at pressures of 1500 p.s.i. or lower.

In the actual application of the method of this invention to. a reservoir, the basic steps are the same as those previously described in connection with the conduct of a long core displacement run. Briefly, having predetermined the operating pressure, as outlined above, the gas is continuously injected into one or more injection wells at a pressure sufiicient to maintain such predetermined pressure at the zone of contact between reservoir oil and the injection gas, such gas injection and pressure main tenance is continued to force the gas through the reservoir and displace oil from the reservoir ahead of the gas, and, simultaneously with such injection fluids are recovered through one or more production wells. Obviously, the surface pressure at the producing wells should be slightly lower than that at the injection wells in order to obtain production and maintain flow through the reservoir. Fluids recovered through the production wells will initially consist of a liquid oil phase with minor amounts of natural gas in solution until breakthrough of the injection gas. Following injection gas breakthrough, the gas-oil ratio will begin to rise due to the presence of increasing amounts of injection gas in the fluid but the produced fluid will still be a single phase fluid gradually changing from liquid to oil to injection gas. Finally, at abandonment gas-oil ratio, conventionally 2. A method in accordance with claim 1 wherein the gas containing nitrogen is substantially pure nitrogen.

3. A method in accordance with claim 1 wherein the gas containing nitrogen is air.

4. A method in accordance with claim 1 wherein the gas containing nitrogen is line gas.

5. The method of recovering oil substantially free of nitrogen from a nondepleted oil reservoir having in communication therewith at least one injection well and at least one production well; comprising, injecting into said reservoir through said injection well and displacing 30,000 s.c.f./STB, the fluid will be substantially an injection gas phase with oil dispersed therein, at which time the process is discontinued. Various patterns of injection and production wells may be selected, depending upon field geometry and other factors well-known to those skilled in the art.

Gases, containing substantial amounts of nitrogen, to be used in accordance with the present invention, include nitrogen, air, flue gas, and other gases containing at least a about 80 percent by volume of nitrogen. ,Flue gas is the preferred gas in accordance with the present invention. It should be apparent that a substantial economic advantage exists in the use of flue gas as opposed to natural gas since combustion of one volume of natural gas will produce nine volumes of line gas. .t-lccordingly, the present invention is particularly adapted to use in areas where natural gas is.in short supply since by simply converting the available naturalgas to flue gas, a ninefold increase in I volume-is obtained. A typical line gas will contain approximately 85 to 90 percent by volume of nitrogen and 15 to 10 percent by volume of carbon dioxide.

Having described the present invention and illustrated the same by specific examples, it is to be understood that such examples are illustrative only and are not to be construed as limiting the present invention, and that equivalent materials and techniques or modifications thereof will be apparent to one skilled in the art and such equivalents and modifications are intended to be covered by the appended claims.

We claim:

1. A method of recovering oil substantially free of nitrogen from a nondepleted oil reservoir havingin communication therewith at least one injection well and at least one production well at least 100 feet apart; oomprising, injecting into said reservoir through said I injection through said reservoir a gas containing at least about percent by volume of nitrogen, the first injected portion of said gas, equal in volume to less than about 0.10 pore volume, also having added thereto C to C hydrocarbons graded in concentration from at least percent by volume of C to C hydrocarbons based on the total volume of gas at the front end of said portion to 0 percent by volume of C to C hydrocarbons at the back end of said portion, at a predetermined pressure below that pressure which will fracture the formation of said reservoir but above the lowest pressure at which said gas is miscible with said oil; and, simultaneously with said injection and displacement, withdrawing through said production well fluid displaced from said reservoir by'said injection gas.

6. A method in accordance with claim 5 whereinthe first injected portion of said gas is equal in volume to about 0.05 pore volume. Y

7. A method in accordance with claim 5 wherein the first injected portion of said gas has added thereto both C; andC to C hydrocarbons in a ratio of C to C to C between about 3 to 1 and 2 to 1 parts by volume.

8. A method in accordance with claim 5 wherein the gas containing nitrogen is substantially pure nitrogen.

9. A method in accordance with claim 5 wherein the gas containing nitrogen is air.

10. A method in accordance with claim 5 wherein the gascontaining nitrogen is flue gas.

References Cited by the Examiner UNITED STATES PATENTS CHARLES E. OCONNELL, Pam Examiner.

UNITED sTATEs PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,209,824 October 5, 1965 Howard A: Koch, Jr., et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, line 51, for "is to" read is to be column 7, line 20, for "liquid to oil" read liquid oil --e Signed and sealed this 24th day of May 1966;

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER- Attesting Officer Commissioner of Patents 

1. A METHOD OF RECOVERING OIL SUBSTANTIALLY FREE OF NITROGEN FROM A NONDEPLETED OIL RESERVOIR HAVING A COMMUNICATION THEREWITH AT LEAST ONE INJECTION WELL AND AT LEAST ONE PRODUCTION WELL AT LEAST 100 FEET APART; COMPRISING, INJECTING INTO SAID RESERVOIR THROUGH SAID INJECTION WELL IN DIRECT CONTACT WITH THE LIQUID OIL IN SAID RESERVOIR AND DISPLACING SAID OIL, THROUGH AT LEST 100 FEET OF TRAVEL PATH IN SAID RESERVOIR, BY DIRECT CONTACT WITH A GAS CONTAINING AT LEAST 80% BY VOLUME OF NITROGEN AT A PREDETERMINED PRESSURE BELOW THAT PRESSURE WHICH WILL FRACTURE THE FORMATION OF SAID RESERVOIR BUT ABOVE THE LOWERST PRESSURE AT WHICH SAID GAS IS MISCIBLE WITH SAID OIL; AND, SIMULTANEOUSLY WITH SAID INJECTION AND DISPLACEMENT, WITHDRAWING THROUGH SAID PRODUCTION WELL FLUIDS DISPLACED FROM SAID RESERVOIR BY SAID INJECTION GAS. 